linux-kernel - Re: [PATCH v8 03/15] futex: Add basic infrastructure for local task local hash.

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Message-ID: <20250207110050.stt_l7KT@linutronix.de>
Date: Fri, 7 Feb 2025 12:00:50 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Juri Lelli <juri.lelli@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org,
	André Almeida <andrealmeid@...lia.com>,
	Darren Hart <dvhart@...radead.org>,
	Davidlohr Bueso <dave@...olabs.net>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Waiman Long <longman@...hat.com>
Subject: Re: [PATCH v8 03/15] futex: Add basic infrastructure for local task
 local hash.

On 2025-02-07 10:41:02 [+0100], Juri Lelli wrote:
> Hi,
> 
> On 05/02/25 09:39, Sebastian Andrzej Siewior wrote:
> > On 2025-02-04 11:34:47 [+0100], Peter Zijlstra wrote:
> 
> ...
> 
> > > Anyway, none of this solves anything when a process has both an active
> > > RT part and an active !RT part (which isn't uncommon AFAICT).
> > > 
> > > Then the RT bits will still get interference from the !RT bits. Do we
> > > want to complicate things and consider that?
> > 
> > I don't think so. The active and inactive are common but it is still the
> > same process so you can expect it. The ugly part is when it is an
> > entirely different task and it is random which one it is.
> 
> Not entirely sure we are thinking about the same situation, but it looks
> like we have cases of RT tasks that are affected by the underlying issue
> this set is about because they make use of libraries. So, in this case
> we have a cross-process (RT/!RT) situation that I am not sure we can
> address sanely. What do you think?

I wouldn't advice to use "unknown" code in a RT application and even
threads. Audit libs before using them and not just collect them.
A lock without PI in your RT thread is not good. A lot of locks, not
knowing the "locked" time, also not good. Things that work most of the
time due to the fastpath and only break from time to time.
Also, a thread does fork() once during start up and things may continue
to be good but may catch up eventually.

> Thanks,
> Juri

Sebastian